1. Field of the Invention
The invention relates generally to three-way catalysts that are able to simultaneously convert nitrogen oxides, carbon monoxide, and hydrocarbons, and specifically to three-way catalyst formulations that contain multi-phase catalysts. The multi-phase catalysts have higher activity than single-phase catalysts with similar compositions. The invention also relates to multi-phase catalysts that additionally contain an oxide layer with precious metal components to increase the activity of the catalyst.
2. Description of the Related Art
Nitrogen oxides, carbon monoxide, and hydrocarbon gases are toxic components in the exhaust gas from internal combustion engines. The simultaneous conversion of the nitrogen oxides, carbon monoxide, and hydrocarbons—“three-way conversion”—is desirable in order to meet emission standards for automobiles and other vehicles. The simultaneous conversion is typically accomplished with catalytic converter units placed downstream from the engine exhaust manifold. In order to achieve an efficient three-way conversion of the toxic components in the exhaust gas, conventional catalytic converter catalysts contain large quantities of precious metals, such as Pd, Pt and Rh, dispersed on suitable oxide carriers in conjunction with other “promoters” on the catalyst. Typically, conventional catalysts use precious metals at concentrations in the range of 30-300 g per cubic foot of catalyst.
Due to the high cost and the limited supply of precious metals, there is a need for a catalyst formulation which efficiently carries out three-way conversion while utilizing precious metals at much lower concentrations than conventional systems. Conventional catalysts with low levels of precious metal generally age more quickly than catalysts with higher levels of precious metal and therefore tend to have short lifetimes. There is a need for three-way catalyst formulations which utilize lower levels of precious metal than conventional catalysts while having a long lifetime.